Wireless power transmitter and method of controlling the same

ABSTRACT

A method and a wireless power transmitter for transmitting charging power to a wireless power receiver are provided. The method includes applying the charging power to the wireless power receiver; determining whether a current value of the charging power is greater than a predetermined threshold; and driving the overcurrent protection circuit upon a determination that the current value of the charging power is greater than the predetermined threshold.

PRIORITY

This application claims priority under 35 U.S.C. §119(e) to aProvisional U.S. Patent Application filed in the USPTO on Oct. 24, 2011,and assigned Ser. No. 61/550,696, the entire content of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a wireless power transmitterand method of controlling the same, and more particularly, to a wirelesspower transmitter for transmitting charging power to a wireless powerreceiver and method of controlling the same.

2. Description of the Related Art

Mobile terminals, such as cell phones, Personal Digital Assistants(PDAs), etc., are powered by rechargeable batteries, and in order torecharge the batteries, the terminals supply electric energy to thebatteries via separate charging devices. Typically, the charging deviceand the battery each having contacting terminals on their respectiveouter surfaces, and are electrically connected to each other via theircontacting terminals.

However, when using such a contact charging method, the contactingterminals are susceptible to be contaminated by dirt because they extendoutward, thus suffering from inappropriate charging. Also, they may notbe properly charged when exposed to moisture.

To address these problems, wireless charging or contactless chargingtechnologies have recently been developed and applied to many differentelectronic devices.

A wireless charging technology using wireless power transmission andreception enables, for example, a battery of a cell phone to beautomatically charged just by placing the cell phone on a charging padwithout a need of a separate charging connector. Such technology iscurrently applied to wireless electric toothbrushes or wireless electricshaver. From the wireless charging technology, the electronic device maybenefited from enhanced waterproof and portable functions because of thelack of need for a wired charging device. And in the coming era ofelectric vehicles, various relevant technologies are expected to be evenfurther developed.

The wireless charging technology has an electromagnetic induction methodusing coils, a resonance method using resonance, and a Radio Frequency(RF)/micro wave radiation method that converts electric energy intomicrowaves for transmission.

Although wireless charging technology has thus far been dominated by theelectromagnetic induction method, due to recent successful experimentsin microwave-based wireless transmission from distances of a few tens ofmeters between devices, it is foreseeable that, in the near future, allelectronic products may be wirelessly recharged anywhere and anytime.

A power transmission method based on the electromagnetic inductiontransfers power between primary and secondary coils. Movement of amagnet through a coil produces an induced current based on which amagnetic field is produced at the transmission end, and the change inthe magnetic field at a receiving end induces a current to generateenergy. This phenomenon is referred to as magnetic induction, and powertransmission methods based on the magnetic induction provide superiorenergy transmission efficiency.

In a resonance method for wireless charging, a professor Soljacic of theMassachusetts Institute of Technology (MIT) suggested a system in whichelectricity is delivered wirelessly, even when the system is a fewmeters away from a charging device, using a resonance-based powertransmission principle based on Coupled Mode Theory. The MIT team'swireless charging system is based on the resonance effect, a physicalconcept where a tuning fork being placed next to a wine glass causes thewine glass to ring with the same frequency. In the resonance methodelectromagnetic waves carrying the electric energy are resonated insteadof sound. Resonant electric energy of electromagnetic waves is directlytransferred only when there is a device having the same resonantfrequency, and the non-used part of the energy is re-absorbed into themagnetic field rather than being dispersed in the air, and thus theresonant electric energy has not been found to be harmful to surroundingmachines or bodies.

A user may arrange the wireless power receiver and the wireless powertransmitter in various locations relative to each other. In this case,impedance at a particular point of the wireless power transmitter mayrapidly change. Such a change in impedance may cause overcurrent to besupplied to the wireless power transmitter. The overcurrent makesoperations of an amplifier in the wireless power transmitter unstable.Furthermore, the overcurrent may cause destroying the power sourceitself due to fire resulting from an excessive current supply.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made to solve theabove-mentioned problems occurring in the prior art, and provide theadvantages and improvements as will be described below. Accordingly, thepresent invention addresses the foregoing problems by providing awireless power transmitter and method of controlling the same to preventovercurrent.

In accordance with an aspect of the present invention, a wireless powertransmitter for transmitting charging power to a wireless power receiveris provided. The wireless power transmitter includes a power supply forsupplying the charging power; a power transmitter for transmitting thecharging power to the wireless power receiver; an overcurrent protectioncircuit for changing an impedance of the wireless power transmitter toprevent a current value of the charging power from exceeding apredetermined threshold; and a controller for determining whether thecurrent value of the charging power is greater than the predeterminedthreshold and controlling to connect the overcurrent protection circuitto the power transmitter upon a determination that the current value ofthe charging power is greater than the predetermined threshold.

In accordance with another aspect of the present invention, a method ofcontrolling a wireless power transmitter for transmitting charging powerto a wireless power receiver is provided. The method includes applyingthe charging power to the wireless power receiver; determining whether acurrent value of the charging power is greater than a predeterminedthreshold; and driving the overcurrent protection circuit upon adetermination that the current value of the charging power is greaterthan the predetermined threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail embodiments thereofwith reference to the attached drawings in which:

FIG. 1 is a diagram illustrating a wireless charging system according toan embodiment of the present invention;

FIG. 2A is a block diagram illustrating a wireless power transmitter anda wireless power receiver according to an embodiment of the presentinvention;

FIG. 2B is a block diagram illustrating a wireless power receiveraccording to an embodiment of the present invention;

FIG. 3 is a flowchart illustrating a method of controlling a wirelesspower transmitter according to an embodiment of the present invention;

FIG. 4A is a circuit diagram illustrating a wireless power transmitterand a wireless power receiver according to an embodiment of the presentinvention;

FIG. 4B is a diagram illustrating a circuit according to an embodimentof the present invention;

FIGS. 5A and 5B are Smith charts illustrating a change in impedance whena wireless power receiver is arranged according to an embodiment of thepresent invention; and

FIG. 6 is a block diagram illustrating a wireless power transmitteraccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION

Hereinafter, embodiments of the present invention are described withreference to the accompanying drawings. In the following description,the same elements may be designated by the same reference numeralsalthough they are shown in different drawings. Further, various specificdefinitions found in the following description are provided only to helpgeneral understanding of the present invention, and it is apparent tothose skilled in the art that the present invention can be implementedwithout such definitions. Further, in the following description of thepresent invention, a detailed description of known functions andconfigurations incorporated herein may be omitted when such adescription may obscure the subject matter of the present invention.

FIG. 1 is a diagram illustrating a wireless charging system according toan embodiment of the present invention.

Referring to FIG. 1, the wireless charging system includes a wirelesspower transmitter 100 and wireless power receivers 110-1, 110-2, . . . ,110-n.

The wireless power transmitter 100 wirelessly transmits respective power1-1, 1-2, . . . , 1-n to the wireless power receivers 110-1, 110-2, . .. , 110-n. Specifically, the wireless power transmitter 100 wirelesslytransmits the power 1-1, 1-2, . . . , 1-n only to wireless powerreceivers authenticated via a predetermined authentication procedure.

The wireless power transmitter 100 establishes an electrical connectionwith at least one wireless power receiver from among the wireless powerreceivers 110-1, 110-2, . . . , 110-n. For example, the wireless powertransmitter 100 transmits the wireless power to the wireless powerreceivers 110-1, 110-2, . . . , 110-n in an electromagnetic waveform.

The wireless power transmitter 100 also performs bidirectionalcommunication with the wireless power receivers 110-1, 110-2, . . . ,110-n. The wireless power transmitter 100 and the wireless powerreceiver 110-1, 110-2, . . . , 110-n process and transmit/receivepackets 201, 202, . . . , 2-n consisting of certain frames, which aredescribed in more detail herein below. The wireless power receivers maybe implemented in mobile communication terminals, Personal DigitalAssistants (PDAs), Portable Multimedia Players (PMPs), smartphones, etc.

The wireless power transmitter 100 wirelessly provides power to theplurality of the wireless power receivers 110-1, 110-2, . . . , 110-n.For example, the wireless power transmitter 100 may wirelessly transmitpower to the plurality of the wireless power receivers 110-1, 110-2, . .. , 110-n based on a resonance method. When the resonance method isadopted by the wireless power transmitter 100, a distance between thewireless power transmitter 100 and the plurality of the wireless powerreceivers 110-1, 110-2, . . . , 110-n may be limited to a maximum of 30m. However, when an electromagnetic induction method is adopted by thewireless power transmitter 100, a distance between the wireless powertransmitter 100 and the plurality of the wireless power receivers 110-1,110-2, . . . , 110-n may be limited to a maximum of 10 cm.

The wireless power receivers 110-1, 110-2, . . . , 110-n charge theirbatteries by receiving wireless power from the wireless powertransmitter 100. The wireless power receivers 110-1, 110-2, . . . ,110-n also transmit, to the wireless power transmitter 100, a signal forrequesting the wireless power transmission, information necessary forwireless power reception, information indicating states of the wirelesspower receivers, and/or control information of the wireless powertransmitter, which are described in more detail herein below.

The wireless power receivers 110-1, 110-2, . . . , 110-n also eachtransmit a message indicating a respective charging state to thewireless power transmitter 100.

The wireless power transmitter 100 includes a display unit, and displaysthe respective states of each of the wireless power receivers 110-1,110-2, . . . , 110-n based on the respective messages received from thewireless power receivers 110-1, 110-2, . . . , 110-n. The wireless powertransmitter 100 also displays an estimate of the time until completionof charging the respective wireless power receivers 110-1, 110-2, . . ., 110-n.

The wireless power transmitter 100 also transmits a control signal toeach wireless power receiver 110-1, 110-2, . . . , 110-n to disable itswireless charging function. When receiving the disable signal from thewireless power transmitter 100, the receiving wireless power receiversdisable their own wireless charging functions.

FIG. 2A is a block diagram illustrating a wireless power transmitter anda wireless power receiver, according to an embodiment of the presentinvention.

Referring to FIG. 2A, the wireless power transmitter 200 includes apower transmitter 211, a controller 211, and a communication unit 213.The wireless power receiver 250 includes a power receiver 251, acontroller 252, and a communication unit 253.

The power transmitter 211 provides power requested by the wireless powertransmitter 200, and wirelessly transmits the requested power to thewireless power receiver 250. Here, the power transmitter 211 suppliesthe power in an Alternate Current (AC) waveform, or may convert thepower in a Direct Current (DC) form into the AC waveform for supply byusing an inverter. The power transmitter 211 may also be implemented inthe form of a built-in battery or a power receiving interface forreceiving power from an outside source and supplying the received powerto other components in the wireless power transmitter 200. The powertransmitter 211 is not limited to the above-described example, but mayalso be implemented in any other such device that provides power in anAC waveform in accordance with embodiments of the present invention.

In addition, the power transmitter 211 provides the AC waveform aselectromagnetic waves to the wireless power receiver 250. The powertransmitter 211 may also include a loop coil to transmit or receive theelectromagnetic waves. When the power transmitter 211 includes a loopcoil, an inductance L of the loop coil may be variable. The wirelesstransmitter 211 is not limited to the above-described examples, but maybe implemented in any device for transmitting or receivingelectromagnetic waves in accordance with embodiments of the presentinvention. The power transmitter 211 may also include a resonancecircuit that includes a loop coil and a capacitor.

The controller 212 controls general operations of the wireless powertransmitter 200. The controller 212 controls the general operations ofthe wireless power transmitter 200 by using a control algorithm, aprogram, or an application read from a storage (not shown). Thecontroller 212 may be implemented in the form of a Central ProcessingUnit (CPU), a microprocessor, or a mini-computer.

The controller 212 also measures a current at a particular point of thewireless power transmitter 200. If the measured current is at leastequal to a predetermined threshold, the controller 212 drives anovercurrent protection circuit, which is described in more detail hereinbelow.

The communication unit 213 communicates with the wireless power receiver250 through a predetermined communication method. The communication unit213 may communicate with the communication unit 253 of the wirelesspower receiver 250 based on Near Field Communication (NFC), Bluetooth,Bluetooth Low Energy (BLE), Wifi, Wifi Direct, Zigbee communication,infrared communication, ultraviolet communication, etc. According toembodiments of the present invention, the communication unit 213 may usethe Institute of Electrical and Electronics Engineers (IEEE) 802.15.4Zigbee communication method. Furthermore, the communication unit 213 mayuse a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)algorithm. Configurations for selecting a frequency and channel for usein the communication unit 213 are discussed in detail herein below. Theforegoing communication methods used by the communication unit 213 arejust provided as examples, and other communication methods may be usedin accordance with embodiments of the present invention.

The communication unit 213 transmits a signal including informationregarding the wireless power transmitter 200. Here, the communicationunit 213 may unicast, multicast, or broadcast the signal. Thecommunication unit 213 receives power information from the wirelesspower receiver 250. The power information includes at least one of acapacity of the wireless power receiver 250, remaining batteryindicator, information indicating a frequency of charging, batteryconsumption, battery capacity, and a battery charge/consumption ratio,for example.

The communication unit 213 transmits a charge function control signal tocontrol a charging function of the wireless power receiver 250. Thecharge function control signal is used to enable or disable the chargingfunction by controlling the power receiver 251 of the wireless powerreceiver 250.

The communication unit 213 may receive signals, not only from thewireless power receiver 250, but also from different wireless powertransmitters (not shown). For example, the communication unit 213 mayreceive, from different wireless power transmitters, a signal for thewireless power transmitter 200.

In the wireless power transmitter 200 of FIG. 2A, the power transmitter211 and the communication unit 213 are separate and use out-bandcommunications, but embodiments of the present invention not limitedthereto. The power transmitter 211 and the communication unit 213 may beintegrated in a single hardware device, and thus the wireless powertransmitter 200 may use in-band communications in accordance withembodiments of the present invention.

The wireless power transmitter 200 and the wireless power receiver 250communicate various signals with each other, and accordinglysubscription of the wireless power receiver 250 to a wireless powernetwork hosted by the wireless power transmitter 200 and chargingprocess through wireless power transmission and reception may beperformed, which is described in detail herein below.

FIG. 2B is a block diagram illustrating a wireless power receiveraccording to an embodiment of the present invention.

Referring to FIG. 2B, a wireless power receiver 250 includes a powerreceiver 251, a controller 252, a communication unit 253, a rectifier254, a DC to DC converter 255, a switching unit 256, and a charging unit257.

The power receiver 251, the controller 252 and the communication unit253, operate in a manner similar to that described above with respect tocorresponding components of FIG. 2A, and accordingly, a furtherdescription of these components is omitted for clarity and conciseness.The rectifier 254 rectifies the wireless power received by the powerreceiver 251 into a Direct Current (DC) format and may be implementedwith bridge diodes, for example. The DC to DC converter 255 converts therectified power to have a predetermined level. For example, the DC to DCconverter 255 may convert the rectified voltage to 5V at its output end259. However, minimum and maximum values of a voltage to be applied tothe front end (input end) of the DC to DC converter 255 may be preset,and the values may be recorded in Input Voltage MN and Input Voltage MAXfields of a request join signal, respectively, which are discussed indetail herein below. Rated voltage and rated current at the output end259 of the DC to DC converter 255 may also be recorded in Typical OutputVoltage and Typical Output Current fields of the Request join signal.

The switching unit 256 connects the DC to DC converter 255 to thecharging unit 257. The switching unit 256 keeps an ON or OFF state undercontrol of the controller 252. The charging unit 257 stores theconverted power input from the DC to DC converter 255 when the switchingunit 256 is in the ON state.

FIG. 3 is a flowchart illustrating method of controlling a wirelesspower transmitter according to an embodiment of the present invention.

Referring to FIG. 3, the wireless power transmitter 200 applies chargingpower to at least one wireless power receiver, in step 5301. In thisregard, the wireless power transmitter 200 applies a predetermined levelof charging power to the at least one wireless power receiver.

The user may change the position of the wireless power receiver 250. Theuser may also arrange the wireless power receiver 250 on the wirelesspower transmitter 200. In such cases, impedance at a specified point ofthe wireless power transmitter 200 may rapidly change. In the process ofchanging the impedance at the specified point of the wireless powertransmitter 200, an overcurrent may be applied to the wireless powertransmitter 200. The wireless power transmitter 200 determines whetherthe applied charging power is greater than the predetermined threshold,in step 5303.

If the applied charging power is less than the predetermined threshold,the wireless power transmitter 200 continues applying the currentcharging power, in step 5301. However, if the applied charging power isat least equal to the predetermined threshold, the wireless powertransmitter 200 drives the overcurrent protection circuit, in step 5305.

The overcurrent protection circuit is described in more detail hereinbelow. For example, the controller 212 controls a switch connected tothe overcurrent protection circuit to be in an “ON” state, if theapplied charging power is greater than the predetermined threshold.Meanwhile, the controller 212 controls the switch connected to theovercurrent protection circuit to be in an “OFF” state, if the appliedcharging power is less than the predetermined threshold.

The overcurrent protection circuit changes the impedance at thespecified point of the wireless power transmitter 200, and thus ensuresthat the charging power applied from the wireless power transmitter isless than the predetermined threshold.

FIG. 4A is a circuit diagram illustrating a wireless power transmitterand a wireless power receiver according to an embodiment of the presentinvention.

Referring to FIG. 4A, a wireless power transmitter includes a node 401.The node 401 is connected to a power supply (not shown) of the wirelesspower transmitter. The node 401 is also connected to a resistor 402 anda comparator 403. Inputs of the comparator are connected to front andrear ends of the resistor 402, respectively. An output of the comparator403 is connected to an input of a comparator 404. Reference powerK_(ref) is applied to the other input of the comparator 404. An outputof the comparator 404 is connected to the overcurrent protectioncircuit. The overcurrent protection circuit includes a first switch 405,a second switch 406, a capacitor 407, and a coil 408. The overcurrentprotection circuit is connected to a ground 409 and a transmissionresonator 418 that includes a coil 418 a and a capacitor 418 b.Specifically, the output of the comparator 404 is connected to bothgates of the first switch 405 and second switch 406. The first switch405 is connected to ground 409 and an end of the capacitor 407. Theother end of the capacitor 407 is connected to an end of the coil 408.The second switch 406 is connected to the other end of the coil 408 andan end of the resonator 418.

In the meantime, the rear end of the resistor 402 and the one input ofthe comparator 403 are connected to an end of a coil 410. The other endof the coil 410 is connected to a node 411. The node 411 is connected toan end of a capacitor 415, an end of a capacitor 417, and a switch 413.A gate of the switch 413 is connected to an amplifier 412, whichreceives a drive signal. The switch 413 is also connected to ground 414.The other end of the capacitor 415 may be connected to an end of a coil416. The other end of the capacitor 417 is connected to ground 414. Theother end of the coil 416 is connected to an end of the resonator 418and 419, the other end of which is connected to ground 414.

The wireless power receiver includes a resonator 421 and 422. Theresonator 421 and 422 are connected to a rectifier 423, which in turn isconnected to ground 424 and a load 425. The resonator 418 and 419transmits the charging power to the resonator 421 and 422.

Although the embodiment of the present invention according to FIG. 4Auses a resonance method in which the transmission resonator 418transmits the charging power to the reception resonator 421, embodimentsof the present invention are not limited thereto. The transmissionresonator 418 may be substituted with a coil, and the charging power maybe transmitted in an electromagnetic induction method in accordance withembodiments of the present invention.

Impedance seen by the coil 416 of the wireless power transmitter isdesignated as Z_(IN). The comparator 404 compares the magnitude of acurrent input from the wireless power supply with the magnitude of thereference power K_(ref).

When the wireless power receiver is arranged in proximity to thewireless power transmitter, the impedance Z_(IN) may change rapidly.

FIG. 5A is a Smith chart illustrating a change in impedance when thewireless power receiver is arranged according to an embodiment of thepresent invention. The portion on the left of the Smith chartcorresponds to a high efficiency/low power region 501.

When the impedance Z_(IN) approaches the high efficiency/low powerregion 501, the amount of charging power applied from the wireless powertransmitter to the wireless power receiver is relatively low while thecharging efficiency is relatively high. The portion on the right of theSmith chart corresponds to a low efficiency/high power region 502. Whenthe impedance Z_(IN) approaches the low efficiency/high power region502, the amount of charging power applied from the wireless powertransmitter to the wireless power receiver is relatively high while thecharging efficiency is relatively low. When the wireless power receiveris arranged in proximity to the wireless power transmitter, theimpedance Z_(IN) changes along with a trace 511 or 512.

When the impedance Z_(IN) changes along with the trace 511, theimpedance may avoid passing through the low efficiency/high power region502. In this case, the application of charging power from the wirelesspower supply (not shown) of the wireless power transmitter may be stablewithout causing the overcurrent. However, when the impedance Z_(IN)changes along with the trace 512, the impedance passes through the lowefficiency/high power region 502. In this case, the charging power fromthe wireless power supply (not shown) of the wireless power transmittermay be higher than a predetermined threshold, thus causing variousproblems (e.g., causing a fire that destroys the amplifier).

The comparator 404 compares the charging power with the reference powerK_(ref), and connects the overcurrent protection circuit to theresonator 418 and 419 if the charging power is greater than thereference power. For example, the comparator 404 outputs a signal tocontrol the first and second switches 405 and 406 to be in the “ON”state when the charging power is greater than the reference power.

When the overcurrent protection circuit is connected to the resonator418 and 419, the impedance Z_(IN) may change. FIG. 5B is a Smith chartillustrating a case in which the change in the impedance after theconnection of the overcurrent protection circuit according to anembodiment of the present invention. Referring to FIG. 5B, when theovercurrent protection circuit is connected, the impedance Z_(IN)changes along with a trace 521.

In this case, the impedance Z_(IN) does not pass through the lowefficiency/high power region 502. Accordingly, the charging powerapplied from the power supply (not shown) of the wireless powertransmitter is kept below a predetermined threshold.

Referring back to FIG. 4A, the overcurrent protection circuit includesthe capacitor 407 and the coil 408, but embodiments of the presentinvention are not limited to this example. The overcurrent protectioncircuit may alternatively include at least one of a capacitor, a coil,and a resistor in accordance with embodiments of the present invention.

FIG. 4B is a diagram illustrating a circuit according to an embodimentof the present invention.

Referring to FIG. 4B, if the overcurrent protection circuit isconnected, the wireless power transmitter, overcurrent protectioncircuit, and wireless power receiver includes a circuit equivalent tothat illustrating in FIG. 4B, including Z_(IN), Z_(OCP) and Z_(LOAD.)Here, Z_(OCP) is impedance of the overcurrent protection circuit andZ_(LOAD) is impedance of the load 425. However, if the overcurrentprotection circuit is not connected, the wireless power transmitter andwireless power receiver include a circuit equivalent to a circuit inwhich Z_(IN) and Z_(LOAD) is connected in series, for example (notshown).

As described above, if the charging power is greater than thepredetermined threshold, the overcurrent protection circuit is connectedto change the impedance. With the impedance change, the impedance is notcontained in the low efficient high power region 502, thus preventing anovercurrent from being applied.

FIG. 6 is a block diagram illustrating a wireless power transmitteraccording to an embodiment of the present invention.

Referring to FIG. 6, a wireless power transmitter 600 includes a powersupply 601, a controller 603, a power transmitter 605, and anovercurrent protection circuit 607. The power supply 601 suppliescharging power.

The power transmitter 605 transmits charging power to a wireless powerreceiver. The overcurrent protection circuit 607 changes impedance inthe wireless power receiver at a specified point, in order to prevent acurrent of the charging power from increasing beyond a predeterminedthreshold. The controller 603 determines whether the charging power isgreater than the predetermined threshold, and if so, the controller 603controls connection of the overcurrent protection circuit 607 to thepower transmitter 605.

The overcurrent protection circuit 607 includes at least one of acapacitor, a coil, and a resistor. The overcurrent protection circuit607 may further include a first switch (not shown) to connect betweenthe controller 603 and the overcurrent protection circuit 607 and asecond switch (not shown) to connect between the overcurrent protectioncircuit 607 and the power transmitter 605.

The controller 603 controls the first and second switches to be in the“ON” state if the charging power is greater than the predeterminedthreshold.

The controller 603 may also include a comparator (not shown) connectedto the power supply 601. A first input of the comparator receives thecharging power from the power transmitter 601 and a second input of thecomparator receives the reference power. The comparator compares inputvalues of the first and second inputs, and outputs a value based on thecomparison. For example, if the input value of the first input isgreater than that of the second input, the comparator outputs a controlsignal to control the first and second switches to be set to the “ON”state.

At least one element of the overcurrent protection circuit 607 (i.e., atleast one of the capacitor, the coil, and the resistor) is configured tohave impedance at a specified point of the wireless power transmitter tobe outside of the low efficiency/high power region.

The controller 603 connects the overcurrent protection circuit 607 tothe power transmitter 605 based on a charging related signal receivedfrom the wireless power receiver. The wireless power transmitter mayfurther include a communication unit (not shown) for receiving thecharging related signal from the wireless power receiver. The controller603 analyzes the received charging related signal to determine whetherthe overcurrent is applied to the wireless power receiver or whether theimpedance belongs to the low efficiency/high power region.

According to embodiments of the present invention, overcurrent caused bya rapid change in impedance may be prevented. Accordingly, unstableoperations of an amplifier or destruction of a power supply by fire maybe prevented. The change in impedance at a specified point of a wirelesspower transmitter may be controlled such that it does not to passthrough a low efficiency/high power region, thereby preventing theapplication of overcurrent.

The controller 603 may connect the overcurrent protection circuit 607 tothe power transmitter 605 if the overcurrent is applied to the wirelesspower receiver or the impedance belongs to the low efficiency/high powerregion.

While the invention has been shown and described with reference tocertain embodiments thereof, it will be understood by those skilled inthe art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the invention as definedby the appended claims.

What is claimed is:
 1. A wireless power transmitter for transmitting charging power to a wireless power receiver, the wireless power transmitter comprising: a power supply for supplying the charging power; a power transmitter for transmitting the charging power to the wireless power receiver; an overcurrent protection circuit for changing an impedance of the wireless power transmitter to prevent a current value of the charging power from exceeding a predetermined threshold; and a controller for determining whether the current value of the charging power is greater than the predetermined threshold and controlling to connect the overcurrent protection circuit to the power transmitter upon a determination that the current value of the charging power is greater than the predetermined threshold.
 2. The wireless power transmitter of claim 1, wherein the overcurrent protection circuit includes at least one element of a capacitor, a coil, and a resistor.
 3. The wireless power transmitter of claim 2, wherein the overcurrent protection circuit further includes a first switch connected between the controller and the overcurrent protection circuit and a second switch connected between the overcurrent protection circuit and the power transmitter.
 4. The wireless power transmitter of claim 3, wherein the controller controls the first and second switches to be in an “ON” state if the current value of the charging power is greater than the predetermined threshold.
 5. The wireless power transmitter of claim 4, wherein the controller includes a comparator connected to the power supply.
 6. The wireless power transmitter of claim 5, wherein a first input of the comparator receives the charging power from the power supply, and a second input of the comparator receives a reference power having the predetermined threshold.
 7. The wireless power transmitter of claim 6, wherein the comparator compares input values from the first and second inputs and outputs a value according to a result of the comparison.
 8. The wireless power transmitter of claim 7, wherein the comparator outputs a control signal to control the first and second switches to be in the “ON” state if the input value of the first input is greater than the input value of the second input.
 9. The wireless power transmitter of claim 2, wherein the at least one element of the capacitor, coil, and resistor prevents impedance at a specified point of the wireless power transmitter from entering a low efficiency/high power state.
 10. The wireless power transmitter of claim 1, further comprising a communication unit for receiving a charging related signal from the wireless power receiver, wherein the controller determines whether to connect the overcurrent protection circuit to the power transmitter by analyzing the charging related signal.
 11. A method of controlling a wireless power transmitter for transmitting charging power to a wireless power receiver, the method comprising: applying the charging power to the wireless power receiver; determining whether a current value of the charging power is greater than a predetermined threshold; and driving the overcurrent protection circuit upon a determination that the current value of the charging power is greater than the predetermined threshold.
 12. The method of claim 11, wherein driving the overcurrent protection circuit includes controlling the charging power to be less than the predetermined threshold by changing an impedance of the wireless power receiver. 